Ceramic block filters are well known in the art. These devices are typically comprised of a parallelepiped-shaped block of ceramic material, through which holes extend the surfaces of which, as well as the exterior surfaces of the parallelepiped, are all covered with conductive material. One surface of the block, typically referred to as the top surface, (which is a surface orthogonal to the axes of the holes through the block) is left unmetallized. The metallization lining the holes and the metallization lining the other surfaces of the block form lengths of transmission line, which are well known in the art as being electrically equivalent to distributed inductive and capacitive elements and are quite useful as filter elements.
These so-called ceramic block filters have been used for some time in two-way communications devices, such as cellular telephones and conventional two-way trunked radios. Their electrical characteristics (such as their operation as either bandpass, lowpass, highpass, or bandstop filters) are controlled in part by the physical dimensions of the block, including particularly its height, as well as so-called top patterning on the unmetallized top surface. Spacing between the holes as well as the diameter of the holes also affects the frequency characteristics of the block filter as well.
It is well known that these block filters out-perform discrete, or lumped, inductive and capacitive elements, particularly so at high frequencies, i.e., at or near 800 Mhz. and above. They are mechanically stable, relatively easy to produce and tune, and are capable of operating at relatively high power levels.
As prevalent as ceramic block filters are, however, it is relatively difficult to design and manufacture a relatively narrow bandwidth bandpass filter as a ceramic block filter. Surface acoustic wave devices, or SAWs, which have also been well known in the art for sometime, are generally characterized as capable of having relatively narrow bandpass frequency ranges, but unfortunately, SAWs are also relatively fragile and difficult to package.
Future two-way communications devices such as cellular telephones and conventional and two-way trunked radios, as well as perhaps selective call receivers (pagers) might benefit from a combination of both ceramic and SAW filter technologies. In a transmitter in a two-way communications device, a ceramic block filter could have the required capability to handle relatively large input power levels while the receiver in such a device might benefit from the increased selectivity provided by a surface acoustic wave device. A packaging structure that accommodates both devices might facilitate the adoption of their combination in such devices more readily.